Electron discharge amplifier



Oct. w, 1939.

H. NELSON 2,175,697

ELECTRON DISCHARGE AMPLIFIER Filed Feb. 26, 1937 Sheets-Sheet 1 I I I II I I I I I I I I -.NVENTOR HERBERT NELSON ATTORN EY @cft. 10, 1939. Eso 2,175,697

ELECTRON DISCHARGE AMPLIFIER Filed Feb. 26, 1937 2 Shets-Sheet 2INVENTOR HERBERT NELSON ATTORNEY Patented Oct. 10, 1939 QFFE ELECTRONDISC i'i Herbert Nelson, Bloomfield, N. 3., assignor, by mesneassignments, to Radio Corporation of America, New York, N. Y., acorporation of Delaware Application February 26, 1937, Serial No.127,828

5 Claims.

My invention relates to electron discharge devices in which thedischarge is modified in certain respects, and more particularly tothermionic amplifiers which have high transconductance and in which acontrol input is multiplied by a relatively large factor.

For various reasons it is desirable in thermionic amplifiers that theratio of transconductance to anode or plate current be high, one reasonbeing that this high ratio decreases the noise to signal ratio of theamplifier. In the conventional thermionic amplifier where the dischargefrom a thermionic cathode is controlled by a grid electrode of the usualopen structure, this high ratio cannot be obtained merely by decreasingthe anode current, as the transconductance varies with the plate currentand becomes low when the output current is small, so that the practicallimit for this ratio is about a transconductance of 2000 micromhos permilliampere of plate current, although it can be shown that under idealconditions a much higher ratio is possible.

Electron multipliers of the secondary electron emission type in which adischarge from an electron source is directed in succession to a seriesof emitters which have a secondary electron emission ratio greater thanunity, may be made to multiply or amplify a primary electron dischargemany thousand fold. A conventional thermionic amplifier with a controlgrid may be used as the source of the modulated discharge in an electronmultiplier, but the transconductance and the anode current aremultiplied by the same factor, so that the ratio betweentransconductance and anode current does not change. Such a device ineffect magnifies the conventional thermionic amplifier without changingits characteristics, and for this reason, among others, the resultsobtained are practically no better, except in magnitude, than thoseobtainable with the conventional thermionic amplifier.

An object of my invention is to provide a meth- 0d and means by which anelectron discharge may at will be modified to produce a discharge inwhich the velocity of the electrons is much more nearly uniform in theregion of a control element than in devices heretofore used.

Another object of my invention is to provide an electron dischargedevice in which the ratio of transconductance to anode current is muchgreater than in the conventional thermionic amplifiers, while the anodecurrent is within permissible limits.

Still another object of my invention is to provide a device in which thetransconductance in- Cl. l79171) creases as the anode current decreases,and the transconductance is high even when the anode current isexceedingly small.

A further object is to provide an improved electron multiplier which hasa thermionic cathode and which is more useful and of highertransconductance than electron multipliers with conventional gridcontrol.

In accordance with my invention I select from the electrons emitted by athermionic cathode and transmit to an anode only those electrons emittedfrom the cathode with an initial velocity less than a predeterminedlimiting velocity. In a desirable form of device a continuous surfaceelement positioned between a thermionic cathode and an anode hasimpressed upon it an effective negative potential with reference to thecathode which is within the voltage range of the cathode emission, andthe electron discharge from the cathode is directed, preferably byelectrostatic and electromagnetic fields, along an unobstructed curvedpath so chosen that immediately adjacent the continuous surface of thatelement there is a region which is a substantial facsimile in electronvoltage or velocity of the region of electron emission at the cathodesurface and from which only those electrons having a velocity exceedingthe retarding voltage between that element and the cathode will be ableto move to the element and thus be abstracted or removed from theelectron stream. The electrons with velocities below the limit areunable to reach the element, and being constrained to follow a path awayfrom the element to an anode or to another collecting electrode by whichthey are collected, constitute the output current, which is the originalelectron stream from the cathode less the electrons which were emittedfrom the cathode with a velocity exceeding a preselected limit andabstracted from the stream by the element. If the element is biased withreference to the oathode to a voltage within the range of velocities ofthe greater part of the cathode emission slight variations in potentialof the element produce such comparatively great changes in ratio betweenthe electrons reaching the element and those failing to reach it thatthe transconductance is very high,

My invention, of which other objects and advantages will be apparentfrom the following description, will best be understood in connectionwith the acompanying drawings in which Figure l is a perspective viewpartly broken away and showing one form of three electrode devicesconstructed in accordance with my invention; Figure 2 is a cross-sectionalong the line 22 of the device shown in Figure 1; Figure 3 is a viewsimilar to Figure 1 of my invention embodied in an electron multiplierof several stages; Figure 4 is a form of device in which the inputsignal causes a voltage swing on the cathode; and Figure 5 showsdiagrammatically a modified form of a device of the kind shown in Figure1.

The particular form of device shown in Figure l as one illustrativeembodiment of my invention is a triode comprising a highly evacuatedenvelope Iii, preferably somewhat elongated, and enclosing the variouselectrodes of the device. Near one end of the envelope is a source ofelectrons, such as an indirectly heated cathode II with a flat oxidecoated surface, preferably in the form of a disc, which is surrounded bya fiat field plate I2 substantially fiush with the flat surface of thecathode. Adjacent the cathode II is a continuous surface sheet electrodeor control element I3 substantially in the plane of the cathode surface,and near the other end of the envelope is a flat anode or outputelectrode I t in substantially the same plane as the element !3 and thefield plate I2. An accelerating electrode I5 extends lengthwise of theenvelope parallel to and equidistant from the field plate I2 and theelectrodes I3 and I4. A magnetic field substantially uniform throughoutthe length of the tube and with its lines of force extendingtransversely of the envelope and parallel to the accelerating electrodeand to the other electrodes I3 and It is produced by means such as anelectromagnet It with elongated pole pieces arranged, as best shown inFigure 2, to extend along opposite sides of the envelope. The electronstream from the cathode I I is concentrated into a beam by the fieldplate I2 which is con- 'nected to be at the same potential as thecathode. The beam is accelerated from the cathode toward theaccelerating electrode I5 and is, as a result of the combined effects ofthe electrostatic field of the accelerating electrode I5 and of thetransverse magnetic field produced by the magnet I6, constrained tofollow a curved or cycloidal path to the vicinity of the control elementI3, and from there a similar path to the anode I4.

The potentials necessary for the operation of the device are supplied bya source of potential such as a battery I'I having its negative terminalconnected to the cathode I I, its positive terminal connected through aload circuit I8 to the anode i i, and some point of positive potentialconnected to the accelerating electrode I5. The element or controlelectrode I3 is maintained at a fixed bias with reference to the cathodeby a variable potentiometer 5%. The significant potential of the elementit with reference to the cathode is the effective negative potential,which may differ from the measured bias potential by the amount of thecontact potential. Usually this effective negative potential is a smallfraction, for exmple, a few tenths, of a volt, and is well within therange of the voltage of the cathode emission. For modulation thepotential on the element I3 may be varied by an input circuit 283.

In operation, the accelerating electrode I5 and anode I i are maintainedat positive potentials and the control element I3 is biased to apotential which is effectively somewhat negative with respect to thecathode. Under the influence of the crossed electrostatic and magneticfields in the tube the electron stream emitted from the oathode willfollow a curved and substantially cycloidal path to the vicinity of theelement I3, and to the anode I4... If the electrostatic field in thespace between the field plate I2, the control element I3, and theaccelerating electrode I5 is uniform, the electrons emitted by thecathode are accelerated so they move toward the electrode I5, anddecelerated as they move against the field of the electrode I 5 andtoward the control element E3. The potentials are so chosen that in thevicinity of the control element the velocity of the electron stream issubstantially zero. As a result, the electrons in the region immediatelyadjacent the control element have substantially the same velocity asthey had when they left the surface of the cathode, consequently, inthis region there is in effect a facsimile of the electron emission atthe cathode. Those electrons in this region which have sufficient energyto overcome the slightly negative field of the element I3 will reach theelement and be collected by it, while the remaining electrons which areunable to overcome the field and reach the element are rejected and willbe directed away from the element it along an arcuate or cycloidal pathto the anode and be collected by it.

The electrons are emitted by the cathode with a Maxwellian distributionof velocities. Assuming that the electrons arrive at the region adjacentthe element IS with the same distribution of velocities, and neglectingthe effect of secondary emission and the reflection of electrons fromthe electrodes I3 and i4, it can be shown that as the fraction of thecathode current which is collected by the element I3 increases and thefraction collected by the anode I4 decreases, the ratio oftransconductance to plate current increases, approaching infinity as theanode current approaches zero. Loss of electrons from the anode bysecondary emission may be substantially prevented by proper design andthe choice of the positive potential on the anode. Reflection of theelectrons from the element I3 presents a more serious problem, but ithas been found that the electron current loss from the element I3 byreflection may be made a small fraction of the electron currentcollected by the element I3, and ratios of transconductance to platecurrent have been obtained which are far higher than are obtainable intriodes of conventional design.

When the ratio of transconductance to plate current is high, theelectron current to the control element IS in the triode shown in Figure1 may be as large or larger than the plate current to the anode Id. Inaccordance with my invention, the triod'e may be combined with anelectron multiplier to increase the plate current to a practical valueWithout a corresponding increase in current to the control element. Oneform of device for this purpose is illustrated in Figure 3, in which arow of three intermedate positive electrodes or anodes Ilia, Mb, and I40, constructed to have a secondary electron emissivity greater thanunity, extends from the control element I3 to the output anode i4.Positive potentials, which increase successively from the intermediateelectrode Ma to the anode I4, are supplied through conventionalconnections such as a resistor 21 connected to the battery Il. In thisform of device the accelerating electrode I5 has supplementary sections55a, I51), and I50 in the plane of the accelcrating electrode andconnected to have successively greater positive potentials. A simpleexpedient is, as shown in Figure 3, to connect each section of theaccelerating electrode to the next following intermediate anode oremitter, so that the accelerating electrode I5 is at the potential ofthe emitter I la, the section I511 is at the po- CJI tential of theemitter Mb, and so on. With this arrangement, the electron streamimpinging upon the secondary emitter I 4a produces a stream of secondaryelectrons which is directed along a trochoidal path to the emitters Mb,I40, and to the anode I4. In this way the output current of the tube maybe multiplied many fold and raised to a practical value, while thecathode current collected by the control element I3 is unchanged and isa very small fraction of the total output of the tube from the anode I4.

In this particular form of device I prefer to form the discharge fromthe cathode into a beam by a field plate 22 with a central aperture, andwhich, instead of being flush with the surface of the cathode, is.positioned slightly above the cathode with the aperture in the fieldplate in' registry with the emitting surface of the cathode. The fieldplate 22 is preferably maintained, about one and a half volts negativewith reference to the cathode by some means such as a battery 23. It isusually sufiicient to maintain a difference of potential of about 100volts between the emitters I 4a, I41), I 40, and the anode I4.

Figure 4 shows diagrammatically a modification of the device in whichthe input impedance is increased to a practical value by applying theinput signal to the cathode II instead of to the control element I3. Inthis particular form of the device the zero potential field plate I2 isannular, surrounds the apertured beam forming field plate 22, and isconnected through a condenser 25 to the control element I3, and may beconnected directly to ground. The input signal 13 impressed by the inputcircuit 20 between the field plate I2 and the cathode. The element I3 ismaintained at the desired potential with reference to the cathode by avariable resistor 24 and is grounded for high frequency through thecondenser 25.

In operation, the resistor 24 is set to produce a potential which causesa suitable fraction of the electron current leaving the cathode to becollected by the element I3, while the remainder, constituting the platecurrent, is collected at the anode M. In determining the sign andabsolute value of this potential on the element I3 consideration must begiven to the so called contact potential which in operation of such adevice appears between the cathode and the cold element I3, and as aresult the element I3 at a static potential which is slightly positivewith reference to the cathode may, during operation of the tube, beeffectively at a slight negative potential. As the potential of thecathode with reference to the field plate I2 varies with the signalinput, the electron current collected by the element I3 also varies andwith it the electron current to the anode Id. The change in currentleaving the oathode voltage is small and may be made a very smallfraction of the change in plate current to the anode, so that a tube ofthis typemay be made which has a high transconductance and acomparatively high input impedance.

Figure 5 shows diagrammatically a modification in which theelectrostatic field in the vicinity of the element I3 is made moreuniform and is protected from distortion due to the presence in thevicinity of the element I3 of a positive electrode such as an emitterplate or an anode, by interposing between the element I3 and the firstpositive electrode to which the discharge flows an auxiliary field plate22a, which is connected inside the tube to the field plate 22 and istherefore also maintained about one and a half volts negative withreference to the cathode. In this form of device the electron beam fromthe cathode flows along a path which is substantially trochoidal, firstto the element I3, where the electrons which exceed a predeterminedvelocity move to the element I3 and are abstracted from the electronstream, after which the rejected electrons then move along a similararcuate path to the vicinity of the auxiliary field plate 22a which issufiiciently negative to prevent the collection of anyelectrons by it.The discharge, substantially unchanged, then moves along another similararcuate or cycloidal path to the emitter I ia upon which the dischargeimpinges with sufficient velocity to produce a copious flow of secondaryelectrons which are directed to and collected by the anode I4.

I have obtained goods results with the form of device shown in Figure 5in which the electrodes of sheet nichrome about 30 millimeters wide,were so set that the center to center spacing of the cathode and theelectrodes I3, 22a, and I la was about 16 millimeters, and the distancebetween the accelerating electrode I5 and the control element I3 about 8millimeters. I have found that good results are obtained with a cathodeI I, about 120 mils in diameter, with a field plate 22 spaced about 15mils from the cathode and having an aperture about 50 mils in diameterin registry with the cathode. With this arrangement good results wereobtained with the accelerating electrode I5 about 100 volts positivewith reference to the cathode, and the emitter plate Ma about 150 voltspositive. In such a device a three volt battery is sufficient for thepotentiometer.

With a device of this kind in which the magnetic field was adjusted togive maximum output current at the anode I4, there was obtained amaximum transconductance of about 10,000 micromhos, and a ratio oftransconductance to plate current roughly ten times as great as isobtained in a conventional triode.

I claim:

1. The method of modifying an electron discharge from an electronemitting cathode which consists in controlling said discharge to producein a region remote from said cathode an electron velocity facsimile ofthe electronic emission at the surface of said cathode, collecting andabstracting from said region substantially all electrons havingvelocities greater than a preselected velocity within the velocity rangeof the cathode emission, rejecting from said region to a point moreremote from said cathode substantially all remaining electrons having alesser velocity, and collecting said remaining electrons at said remotepoint.

2. The method of modifying an electron discharge from an electronemitting cathode which consists in directing the electron discharge fromsaid cathode to an element having an imperforate surface as a beamhaving the same axial and velocity relation to said element as to thecathode and forming in a region adjacent said element an electronfacsimile of the electron velocity emission at the cathode, maintainingthe element at a negative potential with reference to the cathode withinthe range of voltage of initial emission of electrons from the cathode,collecting from said region only those electrons of sufficient velocityto move to said element, and di recting the electrons rejected by saidelement along a path away from said element and from said cathode.

3. In an electron discharge modifier the combination with an electrondischarge device comprising an anode, a continuous surface elementspaced from said anode, a source of electrons for delivering in a regionbeside and in the plane of said element a stream of electrons in adirection transverse to the plane of said element and means forproducing fields to direct said stream to said element along a curvedpath normal to said element at its intersection with said element andthence away from said element from said cathode to said anode, of meansfor maintaining said element at an effective potential of a few tenthsof a volt negative with reference to said source.

4. In an electron discharge modifier the combination of an electrondischarge device comprising an anode, a continuous surface elementspaced from said anode, a source of electrons beside and in the plane ofsaid element and means comprising an accelerating electrode in front ofsaid element and of said source and a magnet producing a magnetic fieldparallel to the surface of said element and transverse to the field ofsaid accelerating electrode for causing the discharge from said cathodeto move toward said element against the field of said acceleratingelectrode and along a path normal to the surface of said element and toarrive at said element with substantially zero velocity, of means forvarying the potential of said element with reference to said source overa range of voltage Within a fraction of a volt.

5. In an electron discharge modifier, the combination with an electrondischarge device comprising an electron emitter for producing a beam ofelectrons, an accelerating electrode in front of said emitter, acontinuous surface element beside said emitter, an anode beyond andexposed to the said element, means for maintaining on said acceleratingelectrode a potential positive with reference to said emitter foraccelerating the electron stream from said emitter, and a magnet forproducing a magnetic field transverse to the field of said acceleratingelectrode and parallel to the plane of said emitter and element todirect said electron stream toward said element and against the field ofsaid accelerating electrode to decelerate said stream to substantiallyzero velocity at said element, of means for maintaining said element ata negative potential with reference to said emitter which effectivelydiffers from the potential of said emitter by a voltage no greater thanthe voltage corresponding to the maximum velocity of emission ofelectrons emitted by said emitter.

HERBERT NELSON.

